Associated PressMayor Michael Bloomberg, left and Police Commissioner Raymond Kelly hold a news conference, Thursday, Sept. 8, 2011 in New York. U.S. officials said Thursday they were investigating a credible but unconfirmed threat that al-Qaida was planning to use a car bomb to target bridges or tunnels in New York City or Washington to coincide with the 10th anniversary of the Sept. 11 attacks, the first tip of an "active plot" around that date.

WASHINGTON -- Just days before the 10th anniversary of the Sept. 11 attacks, U.S. counterterrorism officials were chasing a credible but unconfirmed al-Qaida threat to use a car bomb on bridges or tunnels in New York City or Washington.

It was the first "active plot" timed to coincide with the somber commemoration of the terror group's 9/11 attacks a decade ago that killed nearly 3,000 people.

Counterterrorism officials were investigating the threat throughout the night and into today, as police in New York and Washington said they would increase their already -up staffing levels in light of the recent intelligence.

Law enforcement officials were pursuing three people who may be traveling to the U.S. or who have recently entered the country, based on the detailed information received by the U.S. intelligence community late Wednesday, officials said. The intelligence suggested that al-Qaida planned to car bomb one of the two cities that were hit 10 years ago.

The nation's terror alert level has not changed, but raising it was under consideration last night.

The officials described the threat to The Associated Press on condition of anonymity to discuss sensitive security matters.

Security has been enhanced around the country in the weeks leading up to the 10th anniversary, a date officials have long known could be an appealing time to attack. Law enforcement officials have been particularly wary after information gleaned from Osama bin Laden's compound in Pakistan indicated that al-Qaida had considered attacking the U.S. on this anniversary and other important American dates. Officials have also been concerned that terrorists would see the anniversary as an opportunity to retaliate against the U.S. for killing bin Laden in a military raid in May.

The FBI and Homeland Security Department issued a joint intelligence bulletin Thursday night to law enforcement around the country urging them to maintain increased security and be on the lookout for suspicious activity.

The threat came in a single piece of information and was so specific -- and came at such a time of already heightened alert -- that it could not be ignored, officials said.

"There is specific, credible but unconfirmed threat information," the head of the FBI's New York division, Janice Fedarcyk, said. "As we always do before important dates like the anniversary of 9/11, we will undoubtedly get more reporting in the coming days."

New York Mayor Michael Bloomberg told reporters that police there were deploying additional resources around the city but that New Yorkers should go about their business as usual, and the city's observance of the attacks will go on as planned.

In Washington, law enforcement officials said they were working 12-hour days indefinitely, and Police Chief Cathy Lanier said unattended cars parked in unusual locations risked being towed.

Briefed on the threat information yesterday morning, President Barack Obama directed the U.S. counterterrorism community to redouble its efforts, a White House official said.

As of late yesterday, there were no plans to change Obama's travel schedule on Sunday in light of the threat, the White House said. The president is scheduled to mark the 9/11 anniversary with stops at New York's ground zero, the Pentagon and Shanksville, Pa. He also will deliver remarks Sunday night at a memorial concert at the Kennedy Center in Washington.

Law enforcement officials are checking out all of the details included in the threat, Rep. Peter King (R-N.Y.) said.

"No need to panic," King said. "They have not been able to confirm it yet."

Intelligence officials had not seen any specific or credible threats regarding an attack around the anniversary before Wednesday.

Thursday morning, Homeland Security Secretary Janet Napolitano told reporters that there was "a lot of chatter" around the anniversary of the attacks but that there was no information about a specific threat. Napolitano had been briefed on early interpretations of the threat that morning as intelligence officials were still trying to determine the validity of the information. It later became clear that the threat was specific and credible and could not be dismissed, even though it has not been confirmed.